Abrasion resistant grounding device



June 22, 1937. H. J. CRAWFORD ABRASION RESISTANT GROUNDING DEVICE FiledDec. 14, 1955 I N VEN TOR. I

Patented June 22, 1937 ,PCATENTIV OFFICE aoscszs ABBASION RESISTANTGBOUD IDING DEVICE Harry J. Crawford, Venice, Calif.

Application December 14, 1935, Serial No. 54,521

8 Claims.

This invention relates to an abrasion resistant grounding device andpertains particularly to a device which may be secured to a movingvehicle or the like inposition to engage theground,

i. e., the surface over which said vehicle is travelling, for thepurpose of dissipating accumulated static charges from said vehicle toground in the absence of pyrophoric sparks or the like. The expressionpyrophoric sparks" may be under- 10 stood to mean sparks comparable tothose produced by frictional engagement of an abrasive material with apyrophoric material such as a i'errous metal.

It is common practice to provide a metallic grounding device forvehicles which are carrying volatile explosive or combustible materials,such as gasoline or the like, so that any accumulated static chargeswhich are produced in such vehicles are substantially continuouslydissipated to ground. In general, such grounding devices have consistedofa length of metallic chain or cable provided with a hardened steelcontact member in position to drag upon the ground and effectivelydissipate the aforementioned charges,

and such a device will cause the formation oi a large quantity oipyrophoric sparks by the intermittent frictional engagement of thehardened steel contact member with the ground, and are found to be veryrapidly abraded by such contact, necessitating frequent replacement oithe contact member. Attempts have been made to coat the contact memberwith a hard-surfacing material such as Stellite, but it has been foundthat abrasion has not been sufficiently diminished by such procedure toa point which would justify the added eirpense thereof. In order todiminish or substantially eliminate the production oi? pyrophoricsparks, hard bronze contact members have been substituted for theaforementioned hardened steel contact member in some cases, but it hasbeen found that the abrasion resistance of such a material is not highenough to justify the use thereof. Furthermore, all of theabove-mentioned types of grounding devices make an ob- Jectlonableamount of noise in their incessant impact with a hard-surfaced road.

According to the present invention, I provide a contact element, whichmay be electrically secured to the frame or body oi. a vehicle in anysuitable manner as through the agency of an electrically conductivechain or cable, in which a body element or matrix of a non-metallicmaterial of good abrasion resistant characteristics is provided withmaterial of good electrical con- 55 ducting characteristics incorporatedtherein,

whereby an adequate electrical conductivity is obtained through thelength of said contact element along with the very desirable resistanceto abrasion.

In its preferred embodiment, the device of the present invention isflexible in nature, whereby contact thereof with an abrading surfacewill be free of a pronounced shock, thereby diminishing the abrasiveefifect of such surface thereon, it being appreciated that such a devicewill in general be somewhat elongated in shape and attached to thevehicle body at one end, the other end being allowed to freely engagethe abrad-- lng surface.

One of the principal objects of the invention is to provide, an abrasionresistant grounding device having electrically conductivecharacteristics. A further object of the invention is to provide a,flexible abrasion resistant grounding device having electricallyconductive characteristics, and adapted to yield upon forceful contactwith a solid body in the absence or pronounced shock and noise.

A further object oi the invention is to provide a, device of thecharacter described in which one or more electrically conductive andnon-pyrophoric elements are provided in a matrix of abrasion resistantmaterial.

The device of the present invention, in its simplest embodiment, maycomprise a plurality of electrically conductive elements, such as stripsor particles, oi non-pyrophorio material embedded in a matrix or rubberor other flexible, and preferably resilient, non-metallic material ofhigh abrasion resistant characteristics, but normally of very lowelectrical conductivity, the conductive elements being preferablybrought together at one end of the device in position to provideelectrical contact with one another at the position of or through theagency of a suitable clamping device through which thedevice may beelectrically and structurally secured to a vehicle body. Thecloctrically conductive elements of the device may comprise strips orwires of a preferably nonpyrophoric metal, such as brass, copper, orbronze, preferably electrically inter-connected at one end of the deviceto facilitate electrical and structural attachment of such device to avehicle body, but it will be appreciated that non-metallic electricallyconductive elements such as graphiteimpregnated fabric may beequivalently employed. Furthermore, the abrasion resistant matrix mayconsist of a fabric material, ii. desired; or may comprise arubber-impregnated fabric,

within the contemplation of the present invention. Alternatively, theelectrically conductive characteristics desired in the device may beobtained by incorporating an electrically conductive material in arubber matrix in intimate dispersion, as by loading" rubber with from 5to 30% of flake graphite, metallic foil, metallic flbers, or the like,or such an electrically conductive matrix may be provided withadditional conductive elements such as metallic strips or the like asabove described.

Certain exemplary embodiments of the invention are illustrated in theaccompanying figures oi the drawing and referring thereto:

Fig. 1 is a plan view of an embodiment of my invention which utilizeselectrically conductive screen or wire cloth elements embedded in arubber matrix;

Fig. 2 is a side elevation thereof;

Fig. 3 is an. enlarged partly broken away sectional detail thereof takenon line 3-3 in Fig. 1, illustrating a method of connecting a conductiveelement to a connecting means;

Fig. 4 is a sectional view thereof taken on line 4-4 in Fig. 2;

Fig. 5 is a partly broken away perspective view of an alternativeembodiment of my invention which utilizes metal foil or the like toprovide the desired electrically conductive characteristics to thedevice;

Fig. 6 is a partly broken away perspective view of an alternativeembodiment of the invention which utilizes conductive cords or the likein the relation of conductive elements, provided with a modified form.of attaching means;

Fig. '7 is a view corresponding to Fig. 3, showing a further embodimentof the invention; and

Fig. 8 is a side elevation of a vehicle showing a device of the presentinvention in use.

Further objects of the invention will be specifically set forth in thefollowing description or will be apparent therefrom Referring to Figs. 1to 4, the abrasion resistant grounding device is shown as comprising a.resilient matrix i of rubber or the like, preferably vulcanized rubber,in which a plurality of layers of electrically conductive material suchas screen fabric of copper or other non-pyrophoric metal are embedded asat 2, 3, 4, and 5. The screens are shown in intimate contact at one endof the device with each other and with a suitable connecting means,suchas the bolt 6, which is in.

turn connected to a suitable clevis or clamp means l which may beconnected through a suitable electrical connector, such as a chain orcable, not shown, to the object which is to be grounded. It will beappreciated that the matrix I may either be molded in a single piecewith the conductive material incorporated therein, or it may be formedof sheets with the screens placed between each sheet and thenvulcanized. In either case it is preferable that the mesh of the screenbe sufficiently wide to allow the matrix to interbond therethroughexcept in the event that the matrix completely surrounds such conductiveelements. Furthermore, perforated metal plates may be substituted forthe screens or plates without perforation if the matrix completelysurrounds the plates.

In Fig. 5, I have illustrated an alternative embodiment of my inventionin which a. suitable block'2l may be molded from rubber in which asuitable quantity of electrically conductive particles have beendispersed, as at 22. Said electrically conductive particles, which arethus embedded in the block 2|, may comprise metal foil,

metal turnings, metallic fibers, graphite particles or the like,preferably of a non-pyrophoric character. It will be appreciated that inthis embodiment of my invention suitable electrical connection may bemade to the block 2| by molding a suitable threaded rod, similar to therod 6 shown in Fig. 1, into the matrix at a convenient point, or othersuitable means may be provided for electrical connection, as will besubsequenty brought out.

Fig. 6 shows an alternative type of grounding device which may be formedby embedding strips of conductive material into a suitable rubbermatrix. A suitable matrix is shown at 34 in which conductive strips maybe embedded as at 33. The strips 33 may be of wire, fabric cordsimpregnated with graphite, braided wire and fabric material, or otherconductive material. The conductive elements may be connected togetherin any suitable manner as by twisting, soldering, clamping, or by othersuitable means according to the particular material used for theconductive element so that electrical connection may be made between theobject to be grounded and the conductive element.

As an example of an alternative type of clamping means, the form ofcontact element shown in Fig. 6 is shown with conductive elements ofsuch length as to extend beyond one end of the member or matrix 34. andfolded or bent back over the matrix, a suitable U plate or othersuitable clamping or connecting means being secured to the matrix and tothe conductive elements for the purpose of securing the grounding deviceto the object which is to be grounded and for obtaining a desirableelectrical connection therebetween.

For purposes of simplification only one of the conductive elements 33,designated as 33a, is shown in dotted lines extending through the backof the matrix 34, although it will be appreciated that preferably all ofthe conductive elements may be bent over in the above-described manner,the ends of which are shown terminated at 331;. A suitable clamp or Uplate 35 may then be placed over the matrix 34 and the bent back strips33 and clamped firmly into position by rivets 36 or the like. It will beappreciated that the clamp 35 may also be employed with the embodimentof my invention delineated in Fig. 5. It will further be comprehendedthat the matrices illustrated in Figs. 1 to 4 or in Fig. 6 may be loadedwith a suitable proportion, such as from to 30%, of flake graphite,metal filings, metal foil or other conductive material, or withcombinations thereof, in order to enhance the conductive properties ofthe grounding device. The exact limits of loading material may bedetermined to some extent by the desired abrasion resistance of thematrix in combination with the desired electrical characteristicsthereof and although I find the limits of from 5 to 30% flake graphiteor other material advantageous, I do not choose to be limited by thesepercentages.

Referring to Fig. 7, an alternative form of my invention is shown ascomprising a plurality of fabric sheets 4| each coated with a layer ofelectrically conducting material such as graphite or the like as at 42.The fabric may be a wire cloth fabric consisting of interwoven wire andfabric or other conducting fabric-or it may be a substantiallynon-conducting fabric without departing from the spirit of thisinvention. When using a conducting fabric the conducting coating 42 asat 43. Said thread may beof wire, a mixture till 'able means, as at ofwire and fabric, a metal ribbon, or other thread having electricalconducting properties. I also find it preferable to employ a clamp 44,which may be similar to the clamp 35 in Fig. 6, for the purpose ofproviding mechanical and electrical connection between the vehicle andthe ground ing device. The U plate M may be held firmly in contact withthe sheets 4i and the threads 63 by any suitable means, for example, bybolts 45, or by rivets or the like. The shape of the fabric sheets andthe block formed therefrom may be comparable to the above-describedembodiments or any other convenient form or shape as will besubsequently described. It will be appreciated that the stitching withthe conducting thread need not extend beyond the ends of the clamp M andthat the sheets may be stitched or otherwise intersecured withnon-conducting material for the remaining portion of the device, in anysuitable manner.

Referring to Fig. 8, a vehicle, such as, for can ample, a gasolinetank-truck, is indicated generally at operating on a road or otherground surface 52. The vehicle further comprises a tank 53 and a frameor body structure 5G to which may be attached a grounding device of thisinvention as indicated at Said device may be attached to the truck iiiin any suitable manner, such as, for example, by a clamp fit, which maybe similar to. the clamp l of Fig. l or the clamp of Fig. 6, and by achainor cable or other electrical connector (Bi which may be con nectedat one end to the clamp and at the other end to the frame member Sa l byany suit= it will be appreciated that the connector may be formedintegrally with the device iii; if so desired and that said connectormay also be attached to the vehicle 53 at points other than the framemember 5d.

The grounding devices produced according to my invention may be or anydesired shape; however, I find it advantageous to make them, forexample, about 6 to l2 inches long, one inch thick, and from 2 to 6inches wide. As previously brought out, connection is usually made toone end of the grounding device, and since the dewire or other form ofconnector, ll tend to make connection with the suriace the road orground, at one end. It is for r ason that I prefer to form the devicewith a beveled end as shown at in Figs. 1 and at in Fig. 5, and at 56 inFig. 7, although this beveled edge may be omitted, if desired, iron someembodiments, since the device will form a leveled edge after being usedfor a time.

The greatest wear in any sort of grounding device which is draggedbehind a vehicle usually comes from the impact of the device against theroad or other objects and it will also be noted that these groundingdevices ,do not stay in contact with the ground but tend to bounce alongthe ground striking it intermittently. It is this bouncing phenomenonwhich greatly increases the impact abrasion of these grounding devicesand also tends to produce pyrophoric sparks and make a relatively largeamount of objectionable noise.- It is one ofxthe objects of thisinvention to provide resilient grounding device comprising a pluralityof 'non-pyrophoric contact members in a resilient matrix so that'theabrasion of the contact members will be greatly retarded dueto the factthat the impacts occasioned by such a grounding device wilibe materiallyabsorbed by the matrix, and a relatively small amount of noise isproduced.

It is appreciated that it is not always possible for the highlyconductive elements such as the screens 2, 3, 4, and 5, the foil strips22, and the conductive strips 82 and 33 tomake direct connection withthe ground each time the grounding device strikes the ground, however,these highly conductive elements may strike the ground in manyinstances, and in those instances which they do not strike the ground,there will be a comparatively short path to ground throughthe matrix. Itis further comprehended that the resistivity of the matrix may be offairly high order even when it is loaded to a'maxilnum. However, sincethe path through the matrix from a highly conductive element to groundwill, normally be comparatively short, the actual resistance of th pathwill be comparatively low. Motor vehicles, tank trucks, airplanes andother vehicles, although of comparatively large dimensions, haverelatively small capacities in terms of microfarads and even though theymay be charged to relatively high, voltages with respect to ground theactual quantity of electricity represented is of an extremely low orderand may be dissipated to ground in a relatively short time even thoughthe resistance of the current path is relatively high as compared to adirect metallic contact. a

1 claim:

1. A grounding device for dissipating static charges from a movingvehicle, which comprises: an elongated flexible body element formed ofnon-metallic material of high abrasion resistant characteristics andprovided with electrically conductive material incorporated therein,said electrically conductive material being so placed and being presentin such amount in said element as to provide for electrical conductiontherethrough in the direction of the length of said element; andelectrically conducting attachment means at one end of said element forelectrically and structurally securing the same to a vehicle, said bodyelement being mounted on such vehicle through the agency of saidattachment means in such manner that the other end of said body elementwill engage the ground surrace over which such vehicle is moving.

rality of elongated metallic members embedded in said rubber andextending longitudinally through said body element and electricallyinterconnected at said one end of said element.

4; A grounding device for dissipating static charges from a movingvehicle, which comprises: an elongated flexible body element formed ofnon-metallic material of high abrasion resistant characteristics; aplurality of electrically conductive members disposed within saidelement and extending longitudinally therethrough fromadadjacent one endto and through the other end thereof, said conductive members beingelectrically interconnected at said one end; and electrically conductiveattachment means at said one end in electrical contact with saidmembers,

for electrically and structurally securing said element to a vehicle,said body element being secured to such vehicle through the agency ofsaid attachment means in position to engage the ground surface overwhich such a vehicle is moving solely at the other end of said bodyelement.

5. The device-set forth in claim 4, said flexible element being formedof vulcanized rubber loaded with from 5 to 30% of graphite.

6. The device set forth in claim 4, said flexible clement comprisingalaminated fabric structure, and said conductor members being disposedbetween adjacent layers of fabric.

'7. The device set forth in claim 4, said flexible element comprising alaminated fabric structure and said conductive memberscomprisingconaoeasas ductive layers of graphite disposed betweenadjacent layers of fabric.

8. A non-pyrophoric grounding device for dissipating static charges froma moving vehicle, which comprises: an elongated flexible body elementformed of a resilient non-metallic material of high abrasion-resistantcharacteristics; a plurality of electrically conductive members formedoi a non-pyrophoric metal extending longitudinally within said elementfrom adjacent one end thereof to and through the other end thereof, saidconductive members being electrically interconnected at said one end;and electrically conductive attachment means at said one end of saidelement in electrical contact with said conductive members, forelectrically and structurally securing said element to a vehicle, saidbody e1ement being secured to such vehicle through the agency of saidattachment in position to engage the ground surface over which such avehicle is moving solely at the other end of said body element.

HARRY J. CRAWFORD.

